Aqueous-phase reactions

Under high pressure and temperature, ordinary water behaves very differently. The electrolytic conductance of aqueous solutions increases with increase in pressure. However, for all other solvents the electrical conductivity of solutions decrease with increase in pressure. This unusual behaviour of water is due to its peculiar associative properties.  

Water becomes less dense due to thermal expansion with increase in temperature. The density of water is 1.0 g/cm at room temperature, which changes to 0.7 g/cm at 306 C. At critical point, the densities of the two phases become identical and they become a single fluid, which is called supercritical fluid. The density of water at this point is 0.3 g/cm In the supercritical region, most of the properties of water vary widely. The most important of these is the heat capacity at constant pressure, which approach infinity at the critical point. Also, the dielectric constant of dense, supercritical water ranges from 5 to 20 on variation of applied pressure. 

As the temperature of water increases to the critical point, its electrolytic conductance rises sharply independent of the pressure. This is attributed to decrease in its viscosity over this range. However, near the supercritical point, the conductance begin to drop off 

Following are given some of the reactions which have been carried out in aqueous medium. 

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There are well over 100 gaseous and aqueous phase reactions that can lead to acid formation and more than fifty oxidizing agents and catalysts may be involved. However, in the simplest terms sulfur in fuels is oxidized to SO2, and SO2 in the atmosphere is further oxidized and hydrolyzed to sulfuric acid. Most nitric acid is formed by the fixation of atmospheric nitrogen gas (N2) to NO. (NO and NO2) during high temperature combustion, followed by further oxidation and hydrolysis that produces nitric acid in the atmosphere. These materials can be dry-... 

The parameter a in Equation (43) quantifies any enhancement in the value of ky due to chemical reactivity of the gas in the water. Its value is unity for an unreactive gas for gases with rapid aqueous phase reactions (e.g., SO2) much higher values can occur. 

Not surprisingly, the acid-base balances within the Earth system almost all involve elements of high abundance, i.e., elements that have low atomic number. In many cases, the acidic molecule is an oxygen-containing oxidation product of an element. Table 16-1 lists the main acids and bases in the global environment. The sources of these acids are chemical reactions of reduced forms of the element involved. Both gas and aqueous phase reactions exist for production of acids. 

Opportunity for aqueous-phase reactions Non-toxic, low hazard catalysts Energy-efficient reactions under moderate conditions of pH, temperature, etc. Possibility for carrying out sequential one-pot syntheses... 

C17-0107. Draw Lewis structure sketches showing the aqueous-phase reaction between acetic acid and ammonia. 

When the extraction kinetics is governed by the aqueous phase reaction,... 

Certain aqueous-phase reactions, including some in which acid-base catalysis is involved for this reason, they are considered further in Chapter 8. 

The elementary irreversible aqueous-phase reaction A + B- R + Sis carried out isothermally as follows. Equal volumetric flow rates of two liquid streams are introduced into a 4-liter mixing tank. One stream contains 0.020 mol A/liter, the other 1.400 mol B/liter. The mixed stream is then... 

One hundred moles of A per hour are available in concentration of 0.1 mole/ liter by a previous process. This stream is to be reacted with B to produce C and D. The reaction proceeds by the aqueous-phase reaction. 

Diffusion into the bulk. This is determined by the diffusion coefficient in the liquid (D,). Diffusion within the bulk aqueous phase is much slower than gas-phase diffusion and can be rate-limiting under conditions of high reactant concentrations where the rate of the chemical reaction is high. This appears to have been a problem in some experimental studies of some aqueous-phase reactions relevant to the atmosphere where either bulk solutions or large droplets and reactant concentrations higher than atmospheric were used (Freiberg and Schwartz, 1981). 

Aqueous-phase reaction relative to gas phase concentrations 1/kHRT... 

Relative importance of aqueous-phase reaction to aqueous-phase diffusion ak /2/D /2... 

Schwartz, S. E., Mass-Transport Considerations Pertinent to Aqueous Phase Reactions of Gases in Liquid-Water Clouds, NATO AS1 Series, G6, 416-471 (1986), and in Chemistry of Multiphase Atmospheric Systems (W. Jaeschke, Ed.), pp. 415-471, Springer-Verlag, New York, 1986. 

It is likely that there are as yet ill-defined aqueous-phase reactions in the airborne seawater droplets that release photochemically labile chlorine gases. For example, Oum et al. (1998a) have shown that Cl2 is formed when sea salt aerosols above their deliques-... 

TABLE 7.1 Some Rate and Equilibrium Constants of Aqueous-Phase Reactions of NO and NOza... 

This dependence of the S(IV) concentrations on the pH of the droplet plays a critical role in determining which oxidant dominates the S(IV) oxidation. As discussed in more detail later, the rates of the various aqueous-phase reactions show different dependencies on pH. Some have rate coefficients that increase with... 

Initiation with X = OH has been discussed earlier. Table 8.11 summarizes some of the aqueous-phase HO, chemistry in which OH is generated and reacts in the atmosphere. (Note that the rate constants for some of the aqueous phase reactions shown in Tables 8.10-8.16 depend on such factors as ionic strength see Chapter 5.D.) Involved with this chemistry is that of bicarbonate/carbonate, since OH reacts with these species as well (Table 8.12). It is interesting that, in contrast to the high reactivity of OH toward S(IV) in aqueous solutions, direct reactions of H02/02 with S(IV) do not appear to be important (Sedlak and Hoigne, 1994 Yermakov et al., 1995). 

Extensive data bases for kinetics of aqueous-phase reactions are provided by the U.S. National Institute of Standards and Technology (see NIST, Ross et al., 1994) and by the University of Notre Dame Radiation Laboratory (see Appendix IV for Web site). 

It was assumed that there were no limitations on the rates of oxidation due to mass transport as discussed in detail by Schwartz and Freiberg (1981), this assumption is justified except for very large droplets (> 10 yarn) and high pollutant concentrations (e.g., 03 at 0.5 ppm) where the aqueous-phase reactions are very fast. It was also assumed that the aqueous phase present in the atmosphere was a cloud with a liquid water content (V) of 1 g m-3 of air. As seen earlier, the latter factor is important in the aqueous-phase rates of conversion of S(IV) thus the actual concentrations of iron, manganese, and so on in the liquid phase and hence the kinetics of the reactions depend on the liquid water content. 

FIGURE 9.10 Modified particle modes and growth processes for sulfate particles involving aqueous-phase reactions in low altitude fogs and in higher altitude clouds upon advection of boundary-layer air upwards. (Adapted with permission from Ondov and Wexler, 1998. Copyright 1998 American Chemical Society.)... 

Table 2 Aqueous phase reactions for C02 capture with aqueous MDEA/PZ solution...

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference photooxidation t,/2 > 9.9 d for the gas-phase reaction with OH radical in air, based on the rate of disappearance of hydrocarbon due to reaction with hydroxyl radical (Darnall et al. 1976) koe = 2.77 x 10 11 cm3-molecules-1-s-1 at 299 K (Atkinson et al. 1977 quoted, Carlier et al. 1986) photooxidation t,/5 = 321 d in water, based on a rate constant k = 2.5 x 109 L-mol -s 1 for the aqueous-phase reaction with photochemically produced OH radical of 1 x 10 17 mol-I, 1 (Mill et al. 1980 Giiesten et al. 1981 quoted, Howard 1990)... 

However, increases in the photocatalytic activity have been reported for Ti02 doped by lanthanides, tin, and iron (III) (55). It may be questioned whether these increases could, in fact, arise from the photo-Fenton reaction between the cations located at the surface and the hydrogen peroxide formed in situ, or even possibly because of a partial dissolution of these cations in the case of aqueous-phase reactions. 

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